1. Field of the Invention
The present invention relates to an apparatus for estimating the amount of unburned components in ash in a coal-fired combustion furnace, which monitors the density of in-ash unburned components contained in the burning waste gases to operate the combustion furnace efficiently.
2. Description of the Prior Art
In recent years, with coal having gained its position as a viable alternative energy to oil, a powdered coal burning technology for generator boilers is attracting attention. The technology itself is already an established one, in which the coal is pulverized by a pulverizing mill and the powdered coal, which is separated from coarse grains of coal by a fine/coarse grain separator, is injected in the form of a gas from a burner into a furnace for combustion.
FIG. 4 shows a schematic configuration of a generator boiler using the powdered coal combustion system. In the figure, the coal deposited in a charging mechanism 10 is fed to the pulverizing mill 11 where it is pulverized by rollers 12 to small grains which are separated by a fine/coarse grain separator 13 into coarse grains and fine grains of coal. Two types of fine/coarse grain separator are available: one is a vane type that separates fine grains from coarse grains by changing the angle of vanes and the other is a rotary type that utilizes centrifugal force in separating the fine from the coarse grains of coal.
The powdered fine grains of coal extracted by the fine/coarse separator 13 are fed together with primary air to a burner 15 of the furnace 14. The primary air serves two purposes--drying the powdered coal to make it easier to burn and carrying the powdered coal to the burner. The primary air accounts for 10-30 percent of the amount of air required for combustion. The remainder of the air is supplied as secondary air from around the nozzle of the burner 15. Tertiary air may be supplied to ensure stable ignition or adjust the shape of flame. From an appropriate position in the furnace 14 remote from the burner 15, air for a second-stage combustion (in a two-stage combustion method) is supplied in a direction of propagation of burning gas.
These kinds of air are supplied from a delivery air blower 16 through an air preheater 17, with the amount of second-stage combustion air adjusted by a second-stage air damper 18.
Heat generated by the furnace 14 is transmitted to water in an evaporator tube 19 by radiation or through contact with gases, evaporating the water. The burning gas is passed through the air preheater 17 where the heat of the burning gas is collected, and then discharged by a suction air blower 20 from a stack 21.
In operation of boiler, it is necessary to minimize the amount of noxious emissions from the burning gases such as nitrogen oxides NO.sub.x and sulfur oxides SO.sub.x within an allowable range while at the same time reducing the amount of in-ash unburned components (H.sub.2, CH.sub.4, etc.) that affect the combustion efficiency. Especially with those boilers using coal as a fuel, the rate of combustion is far slower than those of oil and gas and therefore reduces the temperature of the furnace, which in turn increases the amount of unburned substances (H.sub.2, CH.sub.4, etc.) in the ash. The temperature in the combustion furnace is also reduced by the two-stage combustion method, a method intended to reduce the NO.sub.x emissions.
The amount of unburned substances remaining in ash varies greatly depending on the size of coal grains burned by the burner 15. The finer the grain size, the greater the surface area will become through which the coal contacts the air for combustion and the smaller the amount of unburned components that remain in the ash. During boiler operation, it is therefore necessary to monitor the density of in-ash unburned components in the burning waste gases. When there is an increase in the unburned component density in the ash, the fine/coarse grain separator 13 is controlled to extract finer grains of coal to increase the combustion efficiency.
Since the powdered coal combustion is affected by various factors such as fuel ratio, ash components in coal, and grain size distribution, it is very difficult to estimate the in-ash unburned components during the process of combustion. In an effort to make it less difficult to estimate the in-ash unburned components, a technique has been proposed (for example, Japanese Patent Preliminary Publication No. Heisei 2-208412) that provides to the wall of the combustion furnace an inspection window through which the burning flames of the burner are photographed by a camera. Based on the flame images thus obtained, flame temperatures are estimated, and from such data as the flame temperature, the amount of coal supplied, the amount of air supplied and the preheating air temperature, a combustion rate is determined. Using the combustion rate and the amount of ash in the coal, this technique estimates the density of the in-ash unburned components.
However, since, with this conventional technique, an analog video signal from the camera, which is installed on the wall of the combustion furnace, is converted into a digital video signal and digital images of flames are processed to calculate the flame temperature, the apparatus becomes complex. Calculation of the amount of the in-ash unburned components at the outlet of the combustion furnace requires data on temperature distribution and air ratio distribution in the course of combustion, in addition to the flame temperature. It is, however, difficult to measure the overall temperature distribution and air ratio distribution in the entire real combustion furnace.